1) Field of the invention
The present invention relates to an apparatus for controlling the data voltage of liquid crystal display unit to achieve multiple gray-scale, levels and particularly to the apparatus for controlling the data voltage of an active matrix liquid crystal display unit that is useful for a field of flat display panels to achieve digital multiple gray-scale levels.
2) Description of the Related Art
In recent years, information terminals such as personal computers and word processors have become small-sized, high performance, and high function. Many compact information processors of the desktop type, laptop type, and even smaller notebook type and palmtop type are being marketed.
To reduce the size and weight of these information terminals, light-weight and thin display units are required. For this purpose, liquid crystal display units are frequently employed in place of cathode ray tubes (CRTs), which are usually employed for desktop information terminals. The liquid crystal display units include simple matrix liquid crystal display units such as TN (Twisted Nematic) display units and STN (Super Twisted Nematic) display units. Compared with these simple matrix display units, active matrix liquid crystal display units such as TFT (Thin Film Transistor) and MIM (Metal Insulator Metal) display units are frequently used in a field for displaying high quality color images with multiple gray-scale levels, because the active matrix display units are capable of precisely controlling intermediate gray-scale levels, ensuring high contrast ratios, and providing a high response speed, compared with the simple matrix display units.
For the active matrix liquid crystal display units, presently available digital driver ICs for selecting voltage levels for assigning gray-scale levels can handle only eight gray-scale levels. Even driver ICs presently being developed can operate only up to 16 gray-scale levels. To assign multiple gray-scale levels of more than 16 levels, expensive analog drivers must be employed to drive data. This prevents reducing the cost of liquid crystal display units.
Various studies have been made to increase the number of gray-scale levels with proper use of digital driver ICs. For example, a driver having a capacity of eight gray-scale levels may be employed. First, groups each involving eight power source voltages are prepared and temporarily switched from one to another and combined together, to realize a number of gray-scale levels greater than the number of the power sources.
In this case, if a difference between voltages that are combined together is extremely large, transmissivity based on an average of the combined voltages may deviate from a required value, or a gray-scale level may be swapped with another one, to deteriorate the quality of the gray-scale level assignment.
It is required, therefore, to provide an inexpensive digital data driver that can correctly assign multiple gray-scale levels.
Conventional active matrix liquid crystal display units that achieve good display quality include those employing TFTs.
The TFT display units involve many thin film transistors and pixel capacitance between two electrode layers to form a liquid crystal panel, and a display voltage is written in an optional capacitance through corresponding thin film transistors.
Brightness of the pixel depends on the written voltage. For example, "n" pieces of write voltages may be prepared to provide "n" gray-scale levels.
The n write voltages may be generated as follows:
(1) A predetermined constant voltage is amplified through an operational amplifier whose amplification factor is variable in multiple steps (in this case, n steps). (This will be referred to as the first generation method.)
(2) Two constant voltages are divided by resistances into n kinds of voltages, and one of which is selected by a switching element. (This will be referred to as the second generation method.)
Since the first generation method amplifies the predetermined constant voltage through the operational amplifier, it has the problem that a minimum variable width of the amplification factor is determined by the accuracy of the operational amplifier.
Since the second generation method selects one of the two constant voltages through the switching element, it has the problem that the number of the voltage dividing resistances and switching elements must be increased to expand a circuit scale.
Namely, any one of the above methods cannot easily increase the number (n) of generated voltages, and therefore, cannot further increase the number of gray-scale levels.
According to the development of GUI (Graphic User Interface) in recent years, requirements for multiple colors in computer displays have escalated from the conventional eight or 16 colors to 4096 colors with 16 gray-scale levels, or over 260,000 colors with 64 gray-scale levels. At present, however, about 512 colors with 8 gray-scale levels is the maximum, due to the reasons mentioned above. This is completely insufficient to meet the above requirements for multiple colors.